1. Field of the Invention
The present invention relates to a method for removing residual monomers in the manufacture of vinyl chloride resins (hereinafter collectively referred to as PVC). While preventing inclusion of foreign matter (coloring of PVC particles), the invention effectively removes unreacted residual monomers mainly composed of vinyl chloride monomers (hereinafter referred to as VC) which remain in PVC particles and in an aqueous medium when PVC is manufactured.
More particularly, the present invention relates to a method in which residual monomers contained in a PVC slurry (a dispersion mixture of PVC particles, an aqueous medium, and residual monomers) after termination of a polymerization reaction are removed by treatment with steam while generation of bubbles from the PVC slurry is controlled so that the slurry maintains a stable fluid state constantly, thereby achieving effective residual monomer removal, and in which at the same time, foreign matter is prevented from being included in the end product PVC.
The invention also relates to a tower for removing residual monomers.
2. Description of Related Art
PVC has excellent chemical and physical characteristics, and therefore, is used in many fields. Generally, PVC is manufactured by suspension polymerization, emulsion polymerization, block polymerization, or similar methods. Among these methods, suspension polymerization and emulsion polymerization are commonly employed since they provide advantages such that heat of reaction can be easily removed, that the final product of PVC will contain a reduced amount of contaminants, and that PVC after polymerization is particulate, eliminating a granulating stage.
Suspension polymerization and emulsion polymerization are generally carried out by placing VC, an aqueous medium, a dispersant, a polymerization initiator, etc. in a reactor equipped with a stirrer, and stirring the resulting mixture while a predetermined temperature is maintained. It is a general practice that the polymerization reaction is terminated when the conversion factor has reached a range from 80 to 95% which provides an optimum manufacturing efficiency, rather than the reaction being continued until the VC has been converted to PVC by 100%. Therefore, the PVC slurry after undergoing a polymerization reaction contains a considerable amount of unreacted residual monomers.
Since these residual monomers are harmful to humans, care must be taken to avoid cases where they contaminate PVC particles, or where they migrate into water or the air. In other words, residual monomers must be removed and recovered from the PVC slurry as much as possible.
According to ordinary PVC manufacturing methods, an aqueous medium is mechanically separated from the PVC slurry after polymerization, and the aqueous medium and residual monomers which remain in PVC in small amounts are removed by hot air-drying, etc. to obtain powder PVC, which is the end product.
This procedure cannot avoid cases where the gas exhausted from a dryer contains residual monomers, or the obtained powder PVC was contaminated with a small amount of residual monomers. Thus, conventional manufacturing methods involve the risk of jeopardizing the environment.
Japanese Patent Application Laid-open (kokai) Nos. 54-8693 and 56-22305 propose an improved method which uses a tower housing a plurality of perforated trays and having a bottom port for jetting steam into the tower for more effectively removing and recovering unreacted residual monomers from a PVC slurry.
The characteristic feature of this method resides in the perforated trays, each having a perforated bottom plate carrying partition walls thereon to provide zig-zag passages for the treatment of PVC slurry. When the PVC slurry passes the passages formed on the perforated trays, it is exposed to steam jetted from the bottom of the tower through perforation, by which the residual monomers contained in the PVC slurry are evaporated and separated. This type of method for removing residual monomers is called a demonomerizing method, which has become a mainstream of residual monomer removing techniques.
The demonomerizing method, however, has the following drawback. If the flow rate of the PVC slurry running through the passages on the perforated trays is high, the PVC slurry is excessively bubbled by residual monomers evaporated as a result of contact with steam. The excessive bubbling further causes the following drawbacks.
1. PVC slurry flows over the partition walls and is discharged without passing through the passages for treatment (contact with steam), resulting in an admixture with a PVC slurry from which residual monomers have not been sufficiently removed.
2. PVC particles enter the condenser together with the bubbles formed.
3. PVC particles adhere onto the inner walls of the treatment apparatus (tower), and the adhered particles deteriorate after being exposed to steam at a high temperature, eventually making themselves a foreign matter.
Particularly, improved PVC particles having increased porosity, which have been designed to enhance the capability of absorbing plasticizers, contain a large amount of residual monomers within the PVC particles after polymerization. If it is attempted to treat such a PVC slurry by the above-mentioned demonomerizing method, bubbling takes places excessively, and the above itemized shortcomings are even more significant.
The above-described problems may not be encountered if the flow rate of the PVC slurry is reduced, but a reduction in flow rate results in a considerable drop in the treatment efficiency.
In view of the foregoing, the inventors of the present invention carried out extensive studies, and as a result, the following facts were revealed. First, the bubbles generated when a PVC slurry is contacted with steam on perforated trays are in fact bubbles due to boiling of the residual monomers liberated from the inside of PVC particles and the aqueous medium used. Second, the bubbles elevate the apparent volume of the PVC slurry which flows down the perforated trays. Excessive bubbling thus allows the PVC slurry to overflow beyond the partition walls segmenting the passages for treatment. Third, the contact with steam for 5-minutes results in a 70% removal on the weight basis of the residual monomers existing inside the PVC particles, by which it is presumed that the bubbling phenomenon takes place most vigorously in the uppermost plate of the tower for removing residual monomers.
Based on the above findings, the present inventors have devised a specific shape of a tower having perforated trays for removing residual monomers. That is, the inventors have devised a tower having different inner diameters such that the inner diameter of the upper section of the tower is greater than that of the lower section of the tower, the tower having at least two perforated trays therein with at least one plate at the upper section and at least one plate at the lower section of the tower. In addition, at least one hot water ejection device is provided in the tower. When a tower so constructed was used, unreacted residual monomers were effectively separated and recovered from a PVC slurry, and the thus treated slurry contained an extremely reduced amount of foreign matter.
Accordingly, an object of the present invention is to overcome the aforementioned drawbacks involved in the conventional demonomerizing method. This object is achieved by an improved method in which residual monomers contained in a PVC slurry after termination of a polymerization reaction are removed by contacting them with steam while generation of bubbles from the PVC slurry is carefully controlled to keep a stable fluid state of the PVC slurry, and as a result, foreign matter is prevented from being included in the end product PVC. The invention also provides a tower which is used in this method.
More precisely, according to a first aspect of the invention, there is provided a method for effectively removing residual monomers comprising a stage of charging a slurry of a vinyl chloride resin which has undergone a polymerization reaction into a tower for removing residual monomers which comprises the following structural features:
1) the tower has a cylindrical shape with two or more different inner diameters,
2) the inner diameter of the upper section of the tower is greater than that of the lower section of the tower,
3) at least one perforated tray is provided at the upper section of the tower,
4) at least one perforated tray is provided at the lower section of the tower,
5) a flow-down section for flowing down the slurry to the lower perforated tray is provided between the perforated trays,
6) a means for ejecting steam in an upward direction in the tower is provided at the bottom of the tower, and
7) at least one hot water-ejecting device is provided toward the perforated tray in the tower.
Preferably, the method is carried out by using a tower in which the inner diameter of the upper section of the tower is from 1.20 to 5 times that of the lower section of the tower.
According to a second aspect of the invention, there is provided a tower for removing residual monomers which comprises the following structural features:
1) the tower has a generally cylindrical or an inverted circular cone shape with two or more different inner diameters,
2) the inner diameter of the upper section of the tower is greater than that of the lower section of the tower,
3) at least one perforated tray is provided at the upper section of the tower,
4) at least one perforated tray is provided at the lower section of the tower,
5) a flow-down section for flowing down the slurry to the lower perforated tray is provided between the perforated trays,
6) a means for ejecting steam in an upward direction in the tower is provided at the bottom of the tower, and
7) at least one hot water-ejecting device is provided toward the perforated tray in the tower.
Preferably, the inner diameter of the upper section of the tower is from 1.20 to 5 times that of the lower section of the tower.
The above and other objects, features and advantages of the present invention will become apparent from the following description.